"SMS-friend" sometimes can be dangerous because there is possibility that he may be more close to the girl than you as he sharing more secrets with that girl than you. we never know.

some girl tend to be flirty and like to spoilt other couple relationship. we never know. then this will depends how rational is the guy thinks.

* deeply think: a girl will not simply send out their private part picture to a guy that they dunno unless both of them get to know each other quite close. BUT again, some girls are open minded...

For me:
First i will ask him why the girl send him the mms of the private part.
secondly, i'll observe his attitude and behaviour towards me for a period of time.
Thirdly, will not see him for sometimes. will wait and see how is his reaction towards me.

Before i make my decision, i will consider the 4 factors include (how long is the relationship between me and him, trust and maturity, how much my understanding to him)
    1. how long is the relationship between me and him- how stable is it...   
    2. trust- how much i trust on him
    3. maturity- how mature is his thinking
    4. how much my understanding to him- 40%? 50%? 80%? 100%?

after the 4 factors i will make my decision to:

a. break up with him IF:
a-1. he lie on me not only once but continously. i will forgive him for the first time of course.

b. continue to be with him IF:
b-1. he tells the truth but i will not be very close with him for several periods of time.
b-1-1. the truth also depends on what kind of "truth"
b-1-1-1. IF he tells me that the girl send him the mms voluntary n he promise me not to contact to her anymore, and he really no more contact with that girl then i will forgive him.
b-1-1-2. IF he tells me that the girl send him the mms voluntary n i catch them still continue SMS-ing each other then better SPLIT. this GUY no use lo. he doesn't care of your feeling and i BET u he will do it again in future more worst that this case.
anyway, this is just my opinion ya...